Since the discovery of multi-layered organic light-emitting diodes and organic light-emitting devices (hereafter refer to as OLEDs) by Tang and Van Slyke, OLEDs have become the subjects of intensive investigations because of their applications in full-color displays. In simplest form, an organic light-emitting device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light.
These devices are also commonly referred to as OLEDs. Representative of earlier OLEDs are disclosed in Gurnee et al. U.S. Pat. No. 3,172,862 issued on Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050 issued on Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30. pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167 issued on Jan. 9, 1973. The organic layers in these OLEDs, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
Since then, tremendous efforts have been made to enhance the performance of OLEDs from many aspects. The present inventors have made much progress in the region of OLEDs without doubt. A great deal of new materials, especially RGB (red, green, blue)-light-emitting materials, have been designed and synthesized, and then applied in OLEDs. It is well known to use organic electroluminescent compounds as the light-emitting layer in light-emitting diodes. Simple organic molecules such as anthracene, thiadiazole derivatives, and coumarin derivatives are known to show electroluminescence. Semiconductive conjugated polymers have also been used as electroluminescent components, as has been disclosed in, for example, Friend et al, U.S. Pat. No. 5,247,190; Heeger et al., U.S. Pat. No. 5,408,109; and Nakano et al., Published European Patent application 443861. Complexes of 8-hydroxyquinolate with trivalent metal ions, particularly aluminum, have been extensively used as electroluminescent components, as has been disclosed in, for example, Fang et al., U.S. Pat. No. 5,552,678.
Borrows and Thompson have reported that fac-tris(2-phenylpyridine) iridium(Ir(ppy)3) can be used as the electroluminescent component in organic light-emitting devices (Appl. Phys. Lett. 1999, 75, 4.). The performance is maximized when the iridium compound is present in a host conductive material. Thompson has further reported devices in which the light-emitting layer is poly(N-vinyl carbazole) doped with fac-tris [2-(4′,5′-difluorophenyl) pyridine-C′2,N]iridium(III). (see Polymer Preprints 2000, 41(1), 770.)
High-efficiency OLEDs depends heavily on charge injection, transport and recombination. Therefore, hole-blocking materials (HBM) and electron-transporting materials (ETM) are also important in devices. Compared with so many HBMs developed, only few good ETMs were reported so far. Now tris(8-hydroxyquinoline)aluminum (Alq3) is a common ETMs for its good stability in devices, however, due to its smaller electron mobility, large number of holes will come into ETM, and then combination with electron in Alq3 layer. As a result, a small mount of light emission from Alq3 is often observed. Thus, it unavoidably leads to bad color purity. 1,3,5-tris(phenyl-2-benzimidazolyl)benzene (TPBI) is another efficient ETM. However, it is a crystalline compound. It is well-known that crystallization is a serious problem in OLEDs. 2,9-dimethyl-4,7-diphenylphenathroline (BCP) which has high ionization potential (IP) is a very good hole-blocking material (HBM). Nevertheless its bad electron-transporting ability, BCP can not be used as ETM solely.
Therefore, it is very much demanding for good electron-transporting material with high hole-blocking performance to improve efficiency and color purity.